Global warming is having an unprecedented impact on plant phenology and vitality worldwide, potentially leading to significant changes in the food web, carbon, water cycling and ecosystem functions. Environmental drivers explaining spring phenology, mainly including temperature (chilling in late autumn and winter and forcing in late winter and spring) and photoperiod, have been extensively investigated in temperate trees, but these factors have been rarely studied in subtropical forests and their potential effects are therefore relatively unknown. This knowledge gap is especially important to fill because these ecosystems harbor the largest portion of the world's biodiversity. In this study, we sought to test the effects of chilling (low vs. high) and forcing temperatures (20 vs. 25 °C) on spring phenology for seedlings of five subtropical woody species using six climate chambers. We compared forcing requirements for budburst and leaf-out in the different treatments by calculating the number of degree days achieved from the start of the experiment to the time of budburst and leaf-out, and we examined seedlings’ survival under the different treatments. Although the survival of subtropical seedlings was found to be little affected by variation in chilling or forcing, longer chilling duration and warmer forcing temperature led to a lower forcing requirement for budburst, which advanced both budburst and leaf-out. This suggests that the seedlings experienced a non-linear accumulation of forcing with generally a higher efficiency at 25 °C than at 20 °C. Interestingly, shorter exposure to chilling conditions disrupted the sequence of budburst / leaf-out timings among the study species. This study confirms that the sensitivity of spring leaf phenology to forcing and chilling is not only found in temperate perennial plants but also in subtropical trees that undergo a dormancy period. It offers new perspectives for a comprehensive analysis of subtropical plant phenology in response to global climate change.

全球变暖对全球植物物候和生命力产生了前所未有的影响，可能导致食物网、碳、水循环和生态系统功能发生重大变化。解释春季物候的环境驱动因素，主要包括温度（深秋和冬季的寒冷以及晚冬和春季的强迫）和光周期，已在温带树木中进行了广泛研究，但在亚热带森林中很少研究这些因素，它们的潜在影响是因此比较不为人知。填补这一知识空白尤为重要，因为这些生态系统拥有世界上最大的生物多样性。在这项研究中，我们试图测试寒冷（低与高）和强制温度（20 与 25 °C) 使用六个气候室研究五种亚热带木本植物幼苗的春季物候。我们通过计算从实验开始到发芽和落叶时间达到的度日数，比较了不同处理对发芽和落叶的强制要求，并检查了不同处理下幼苗的存活率。尽管发现亚热带幼苗的存活率受低温或胁迫变化的影响很小，但较长的低温持续时间和较高的胁迫温度导致对发芽的胁迫要求较低，从而促进了发芽和落叶。这表明幼苗经历了非线性的强迫积累，在 25 °C 时的效率通常高于 20 °C 时的效率。有趣的是，较短的寒冷条件暴露扰乱了研究物种的发芽/出叶时间序列。这项研究证实，春叶物候对强迫和寒冷的敏感性不仅存在于温带多年生植物中，也存在于经历休眠期的亚热带树木中。它为综合分析亚热带植物物候响应全球气候变化提供了新的视角。